We analyzed the ability of ERF to repress non-ets-associated tumorigenicity. We have previously shown that ERF can suppress mos-, met- and partially src-associated tumorigenicity. Our analysis indicates that this effect is partially due to the transcriptional repression of the transforming oncogene. However, Ha-ras-induced transformation was not effected by ERF. We utilized the phosphorylation-deficient mutants of ERF to determine if activation of the phosphorylation cascade by ras can inhibit ERF function. ERF genes mutated at various phosphorylation sites were capable of repressing ras transformation in the NIH3T3 system. The reduced transformation was determined by morphological changes and tumorigenicity in nude mice. Both the levels of ERF mutants and ras were constant among the various cell lines, indicating that ERF phosphorylation was the determining factor for this effect. We also introduced ERF in rat embryo fibroblast cell lines that express a temperature-sensitive mutant of mos. In this way, we can study the effect of the mitogen-associated protein kinase (MAPK) cascade on the ERF function. Finally, we established cell lines that utilize the tetracycline repressor system and we generated constructs that place ERF and ets mutant under the tetracycline operator control in order to study the suppressor ability of ERF independent of the clonal problem that may arise from the selection process.